The so-called mark of the beast is referenced only in the book of Revelation: 13:16,17; 14:9,11; 16:2; 19:20; 20:4. (In the New American Bible, the phrase “mark of the beast” appears just once, in 16:2.) The beast is the Antichrist, or even Satan himself. By demanding people accept the mark, the beast tries to force them to renounce the faith. The mark designates those who have committed apostasy.

Scholars generally assume the apostle John wrote the Book of Revelation late in the first century. In that time, codes for names were sometimes created by assigning a number to each of the letters of the name. The number 666 is commonly regarded as a cryptogram for Nero, Roman emperor A.D. 54-68. He was responsible for the first outbreak of murderous persecution of Christians. The numerical values assigned to the letters of his name written in Hebrew add up to 666. Some ancient manuscripts of the Book of Revelation give the number of the beast as 616. This would be the cryptogram for the name of Nero spelled in Latin.